If you just bump the end of the stem of that stuck valve a few times with a piece of broom-handle exactly in line with the valve stem, tapped carefully with a mallet, it may free up. You'll find you can remove the pushrod and turn the rocker a bit to make this possible - don't hit the top of the rocker, even with a broom-handle. (You could tell I used to work on English cars in the long-ago, couldn't you? The British specialised in making lousy piston rings, so we tended to need ways to deal with carbon build-up.)
Valves don't stick without a reason, and that GXV140 having been a very favoured model among mowing contractors, the reason is likely to be a build-up of carbon on the stem, caused by oil-burning. If there are external indications of oil-burning you are going to have to replace the piston rings anyway, and it would be kinder to just take the head off in the first place so you can remove and clean the valves rather than unstick them.
The GXV140 went out of production in 2003, and I believe the last of the HRU195 & to a lesser extent HRU215 mowers that the contractors loved so well was sold new in 2007, so your machine probably has one or two hours on it by now. You can be lucky - mine had its base ruined before the engine wore out, so it's still pretty good. A nice engine. Yours being self-propelled will have a cast iron camshaft I think. The plastic camshaft in the push-propelled model was one of the few things I don't really like about that engine, but I haven't personally seen a worn out plastic camshaft in an engine that hadn't been pulled down previously.
